This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present techniques. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Mining operations typically utilize an extraction process that results in a product and a nonproduct stream. The nonproduct stream is often referred to as “tailings.” When a liquid is included within the extraction process, this can result in fluid tailings that are to be stored in suitable enclosures. In the case of oil sands mining, these tailings form tailings ponds in which fine particles settle over a period of several years to form a stable suspension of 30 weight percent (wt %) solids in water. This suspension is known as mature fine tailings (MFT). The accumulation of MFT on a massive scale has resulted in legislation in Alberta, Canada to form trafficable tailings deposits, i.e., to dewater tailings and ultimately allow reclamation activities upon mine closure.
At present, there are several techniques for dewatering tailings, but they have relatively high costs. These high costs are driven by materials handling issues, technology operating issues, and capital costs, as well as the cost of setting aside Designated Disposal Areas (DDA) of the mine site for tailings dewatering activities. Mining operations that produce plentiful fluid tailings may involve the dedication of an area of land of significant surface area to DDAs. This can sterilize ore or pose higher costs for extraction due to subsequent materials handling.
Currently, the leading technologies for dewatering tailings include a composite tailings (CT) process, a centrifuge process, a thickened tailings process, and an in-line flocculation process. The CT process works by combining mature fine tailings (MFT) and sand with a coagulant to form a non-segregating mixture. Tailings are often flocculated to form thickened tailings, instead of mature fine tailings, and then used in the production of composite tailings. In either case, the mixture is placed in a deposition cell and allowed to dewater over time. Unfortunately, composite tailings are sensitive to shear, which causes sand to separate from fines, resulting in “off-spec composite tailings.” Because off-spec composite tailings dewater very slowly, off-spec composite tailings are stored in tailings ponds. Because of the addition of sand, the volume of composite tailings is often much greater than the volume of the original MFT, resulting in higher storage costs for off-spec composite tailings that dewater slowly.
The CT process fails when desegregation of the sand and fines occurs. Such desegregation may cause the fines to float to the top as the sand sinks to the bottom. The CT process succeeds when the sand stays within the viscous fines fluid and adds extra weight to the fluid, inducing dewatering and consolidation. When the sand sinks through the fluid, consolidation of the fines cannot be further induced by the effective stress of the sand load.
Centrifuges are commercially available devices that dewater tailings based on density differences. Rotation causes centripetal force, which induces higher density material to move to the edges, while lower density material, e.g., water, moves to the middle. This separation enables the densification of tailings. Often, centrifugation is combined with a flocculent treatment to make the solids more readily separable. Centrifuges have high operating and capital costs, and do not scale well for deployment in large applications. As a result, many centrifuges may be used for a particular application, resulting in high capital and maintenance expenses.
The thickened tailings process is becoming more common in mining applications. A thickener is a conically-shaped vessel in which tailings are allowed to settle and compact. The thickener compaction zone enables dewatering to occur, but the rates of compaction are often balanced with the degree of compaction and the ability to continue to flow. Thickeners usually make use of flocculation, and often have a rake to provide shear of the consolidating zone. The rake shears the zone to enhance dewatering. Thickeners are often enormous vessels, which contributes to their capital costs. The need for flocculants for treatment also contributes to high operating costs. Furthermore, the limitation of having to move material from the bottom of the thickener limits their application for final dewatering processes.
The in-line flocculation process involves passing tailings through a pipe. While they flow, the tailings are contacted with a flocculant. This flocculant mixes with the tailings in the pipe. Thus, the inflow to the pipe can be untreated tailings, while the outflow is flocculated tailings. This technology often involves higher dosing of flocculant than thickeners, but has the advantage of not requiring a large vessel. Thus, this technology typically has high operating costs and low capital costs.
The above technologies are often coupled with a strategy for deposition of the tailings. Tailings can be deposited in thick lifts, e.g., those that are on the order of about 3-10 meters. If tailings behave like a fluid rather than a solid, thick lifts are contained within a structure, such as a dam, dyke, or toe system. One strategy for enhancing drainage in thick lift deposition involves the application of dug trenches around the perimeter of the deposit, while another strategy involves installing wick drains—typically, a strip or tube of fabric or porous material which allows accelerated capillary, pressure, or gravity drainage of liquids from wet porous solids (e.g., soil).
Thin lift deposition is another option. However, thin lifts, e.g., those that are less than about 1 m, use large tracks of land in order to distribute tailings on dry ground, so that the tailings may dewater before the next lift is deposited. Tailings can be deposited above the water table to enable dewatering by atmospheric drying, drainage, and consolidation, or below the water table, which leverages consolidation but not atmospheric drying.
As mentioned above, wicks have been proposed and used to aid dewatering of thin lift deposition and thick lift deposition methods, which involve laying out wet but semi-consolidated tailings over an area and allowing time for water to drain out and/or to evaporate out of the tailings. The use of wicks to accelerate dewatering however has some drawbacks for these applications. For example, wick placement requires the use of specialized heavy equipment. Furthermore, wicks are not utilized prior to the material gaining at least some strength, i.e. become more solid-like than fluid-like in rheology. This can be because the equipment must drive over the material in order to place the wick, or because the wick would otherwise fill up with high liquid content slurry which could render the wick useless. Also, wicks are often placed fairly closely spaced. All of the above contribute towards the cost of wick installation becoming a large portion of the price of utilizing wick drains in dewatering.